I love the bag, but there are negatives to it.
If you load in down with more than say 15 pounds, you need to use the waist strap to take the weight off your shoulder to avoid fatigue. I imagine this is the issue with any sling/messenger bag. The weight is not balanced since it rides on just one shoulder.
That said, I have ran 5 miles with about 20 pounds loaded in it, and I didn't have any issues while wearing the waist strap. There is no way I could have done that with just the shoulder strap though.
I carry a Sony Viao 15 inch in the bladder compartment with no issues. Fits perfectly. I have read about people fitting 17 inchers in the main compartment, But haven't tried it.
I wore it today, and it was 105. I didn't notice the bag being any hotter than the inferno I was already in.